The plant pathogenic bacterium Pseudomonas syringae is noted for its diverse and host-specific interactions with plants. A specific strain may be assigned to one of at least 40 pathovars based on its host range among different plant species and then further assigned to a race based on differential interactions among cultivars of the host. In host plants the bacteria typically grow to high population levels in leaf intercellular spaces and then produce necrotic lesions. In nonhost plants or in host plants with race-specific resistance, the bacteria elicit the hypersensitive response (HR), a rapid, defense-associated programmed death of plant cells in contact with the pathogen (Alfano & Collmer, J. Bacteriol. 179:5655–5662 (1997)). The ability to produce either of these reactions in plants appears to be directed by hrp (HR and pathogenicity) and hrc (HR and conserved) genes that encode a type III protein secretion pathway and by avr (avirulence) and hop (Hrp-dependent outer protein) genes that encode effector proteins injected into plant cells by the pathway (Alfano & Collmer, J. Bacteriol. 179:5655–5662 (1997)). These effectors may also betray the parasite to the HR-triggering R-gene surveillance system of potential hosts (hence the avr designation), and plant breeding for resistance based on such gene-for-gene (avr-R) interactions may produce complex combinations of races and differential cultivars (Keen, Annu. Rev. Genet. 24:447–463 (1990)). hrp/hrc genes are probably universal among necrosis-causing gram-negative plant pathogens, and they have been sequenced in P. syringae pv. syringae (Psy) 61, Erwinia amylovora Ea321, Xanthomonas campestris pv. vesicatoria (Xcv) 85-10, and Ralstonia solanacearum GMI1000 (Alfano & Collmer, J. Bacteriol. 179:5655–5662 (1997)). Based on their distinct gene arrangements and regulatory components, the hrp/hrc gene clusters of these four bacteria can be divided into two groups: I (Pseudomonas and Erwinia) and II (Xanthomonas and Ralstonia). The discrepancy between the distribution of these groups and the phylogeny of the bacteria provides some evidence that hrp/hrc gene clusters have been horizontally acquired and, therefore, may represent pathogenicity islands (Pais) (Alfano & Collmer, J. Bacteriol. 179:5655–5662 (1997)).
Virulence effector proteins delivered to or into host cells by type III secretion systems are key factors in the pathogenicity of many bacteria, including animal pathogens in the genera Salmonella, Yersinia, Shigella, and Escherichia, and plant pathogens in the genera Pseudomonas, Erwinia, Xanthomonas, Ralstonia, and Pantoea (Galán & Collmer, Science 284:1322–1328 (1999)). In plant pathogens, the type III secretion machinery is referred to as the hypersensitive response and pathogenicity (Hrp) system because secretion mutants typically lose their ability to elicit the defense-associated hypersensitive response in nonhost plants and to grow parasitically or be pathogenic in host plants (Alfano & Collmer, J. Bacteriol. 179:5655–5662 (1997)). These phenotypes demonstrate the importance of the Hrp system in bacterium-plant interactions, and global identification of effectors will be important for understanding the pathogenesis of bacteria that use type III secretion systems. Unfortunately, several factors have hindered searches for type III effector genes. These factors include: (i) effectors are often redundant with mutants having only subtle phenotypes; (ii) with few exceptions (see e.g., Miao & Miller, Proc. Natl. Acad. Sci. USA 97:7539–7544 (2000)) motifs that can identify proteins as substrates for type III secretion have not been recognized (Lloyd et al., Mol. Microbiol. 39:520–523) (2001); (iii) many effectors show no similarity to known proteins; and (iv) some pathogens have multiple type III secretion systems which deliver different sets of effectors (Cornelis & Van Gijsegem, Annu. Rev. Microbiol. 54:735–774 (2000)). Thus, a complete inventory of type III effector genes is lacking for any pathogen, although it seems that pathogens such as Salmonella may have many such genes (Worley et al., Mol. Microbiol. 36:749–761 (2000)).
Plant pathogen type III effector proteins are mostly designated Avr or Hop, depending on whether their primary phenotype involves plant reaction or secretion behavior. Many effectors were initially discovered through their ability to betray the pathogen to the host R (resistance) gene surveillance system, thereby rendering the pathogen avirulent on a test plant (Keen, Annu. Rev. Genet. 24:447–463 (1990)). Over 25 effector genes have been identified by Avr or Hop phenotypes in various P. syringae pathovars and races (Vivian & Arnold, J. Plant Pathol. 82:163–178 (2000); Alfano et al., Proc. Natl. Acad. Sci. USA 97:4856–4861 (2000)). The encoded effectors seem to determine both basic pathogenicity and host range, but the number of such proteins produced by any single strain has not been systematically investigated. P. s. tomato DC3000 is known to carry at least three avr genes, avrPto (Ronald et al., J. Bacteriol. 174:1604–1611 (1992)), avrPtoB (Kim et al., Cell 109:589–598 (2002)), and avrE (Lorang & Keen, Mol. Plant-Microbe Interact. 8:49–57 (1995)), with the latter being in the Hrp pathogenicity island along with five other candidate effector genes (Alfano et al., Proc. Natl. Acad. Sci. USA 97:4856–486 (2000); Lorang & Keen, Mol. Plant-Microbe Interact. 8:49–57 (1995)).
The present invention is a further advance in the effort to identify, clone, and sequence Avr and Hop proteins or polypeptides from plant pathogens.